DESCRIPTION

dup()
uses the lowest-numbered unused descriptor for the new descriptor.

dup2()
makes newfd be the copy of oldfd, closing newfd
first if necessary, but note the following:

*

If
oldfd
is not a valid file descriptor, then the call fails, and
newfd
is not closed.

*

If
oldfd
is a valid file descriptor, and
newfd
has the same value as
oldfd,
then
dup2()
does nothing, and returns
newfd.

After a successful return from one of these system calls,
the old and new file descriptors may be used interchangeably.
They refer to the same open file description (see
open(2))
and thus share file offset and file status flags;
for example, if the file offset is modified by using
lseek(2)
on one of the descriptors, the offset is also changed for the other.

The two descriptors do not share file descriptor flags
(the close-on-exec flag).
The close-on-exec flag
(FD_CLOEXEC;
see
fcntl(2))
for the duplicate descriptor is off.

dup3()
is the same as
dup2(),
except that:

*

The caller can force the close-on-exec flag to be set
for the new file descriptor by specifying
O_CLOEXEC
in
flags.
See the description of the same flag in
open(2)
for reasons why this may be useful.

*

If
oldfd
equals
newfd,
then
dup3()
fails with the error
EINVAL.

RETURN VALUE

On success, these system calls
return the new descriptor.
On error, -1 is returned, and
errno
is set appropriately.

ERRORS

EBADF

oldfd
isn't an open file descriptor, or
newfd
is out of the allowed range for file descriptors.

EBUSY

(Linux only) This may be returned by
dup2()
or
dup3()
during a race condition with
open(2)
and
dup().